WP Report Post

Report Post is a highly customizable plugin that lets your visitors to report posts or pages with inappropriate content. All these reports are displayed as a table in your Administrator section so you can decide what to do next: edit contents, unpublish posts/pages, or just delete these reports. The plugin was designed to work in both automatic and manual modes. In automatic mode, the link to report will be added to post's meta box. In manual mode, you can place the link, button or image anywhere you want in templates.

Features:

Easy to use - you can simply activate the plugin and it will do the thing